1. Field of the Invention
The present invention relates to a magnetic roller, a development device, and a processing cartridge for use in an image forming apparatus such as a copying machine, a facsimile, and a printer, and to an image forming apparatus. More particularly, the present invention relates to a development device for developing an electrostatic latent image on a photoreceptor drum, so as to form a toner image by feeding developer carried by a development sleeve including a non-magnetic cylindrical body onto a development area where the photoreceptor drum faces the development sleeve at intervals, and a magnetic roller for use in the development device. Moreover, the present invention relates to an image forming apparatus including the development device and a processing cartridge.
2. Description of Related Art
Conventionally, in an image forming apparatus having a photoreceptor drum as an image carrier, such as a copying machine, a printer, and a facsimile based on an electrongraphic method, for example, an image is transferred by the following operations. At first, a photosensitive layer of the photoreceptor drum is charged by a charging roller. Next, an electrostatic latent image is formed by exposing the photoreceptor drum to a laser beam from a laser scanning unit, and the electrostatic latent image is developed by toner, and then an image is transferred onto transfer paper as a transfer material.
A development device having a so-called two-component development process using developer mixed non-magnetic toner with magnetic carriers is used in the above-described image forming apparatus. The development device having the two-component development process includes a developer carrier having a columnar development sleeve and a magnetic roller disposed in the development sleeve.
The magnetic roller includes a body part having a circumferential face buried with a magnet. A plurality of magnetic poles is formed by the magnet. In this case, the magnet for forming each of the magnetic poles is formed in the shape of a bar, for example. Especially, a development main magnetic pole for napping the developer in the shape of a brush is formed in a part corresponding to the development area part of the surface of the development sleeve. The developer napped in the shape of a brush by the magnetic pole moves in the circumferential direction by rotating at least either the development sleeve or the magnetic roller. In order to easily feed the developer, a surface roughening process such as a grooving process and a sandblast process is generally conducted on the surface of the development sleeve. The surface roughening process such as the grooving process and the sandblast process is conducted for preventing a decrease in the image concentration caused by the developer slipping and remaining on the surface of the development sleeve rotating at a high speed.
FIG. 20 illustrates a development device of related art. A development device 3′ includes a developer carrier 4′ for feeding developer to a development area facing a photoreceptor drum 23′, and developing an electrostatic latent image formed on the surface of the photoreceptor drum 23′, so as to form a toner image. In addition, the developer carrier 4′ includes a cylindrically formed development sleeve 5′ and a magnetic roller 6′ housed in the development sleeve 5′ for forming magnetic fields, so as to nap the developer onto the surface of the development sleeve 5′. In the developer carrier 4′, when napping the developer, the magnetic carriers constituting the developer are napped onto the development sleeve 5′ along the magnetic lines generated by the magnetic roller 6′. The toner constituting the developer is adhered onto the napped magnetic carriers.
Such a development device 3′ includes a developer tank 311′ for containing the above-described developer, a screw-shaped agitation member 312′ for agitating the developer in the developer tank 311′, and a developer control member 32′ for equalizing the amount of developer transferred onto the developer carrier 4′.
In the development device 3′ illustrated in FIG. 20, the developer tank 311′ includes a pair of developer tanks 311a′, 311b′ and the agitation member 312′ includes a pair of agitation members 312a′, 312b′. The developer in the development device 3′ moves in the developer tank 311′ in the axial direction of the agitation member 312′. The toner supplied from one end portion of one developer tank 311a′ on the side furthermost away from the developer carrier 4′ is agitated with the developer by one agitation member 312a′ while being fed to the other end portion of the one developer tank 311a′ along the axial direction of the one agitation member 312a′. The developer moves into the other developer tank 311b′ close to the developer carrier 4′ from the other end portion of one developer tank 311a′. The developer moved into the other developer tank 311b′ close to the developer carrier 4′ is transferred onto the surface of the development sleeve 5′ by the magnetic force of the magnetic roller 6′. After that, the amount of developer is uniformed by the developer control member 32′, and then is fed to a development area 41′ where the photoreceptor drum 23′ faces the developer carrier 4′ at intervals. Then, the developer develops the electrostatic latent image formed on the photoreceptor drum 23′, so as to form a toner image.
Recently, such an image forming apparatus has been increasingly colorized and downsized. Since four development devices are generally built in a color copying machine, it is necessary to downsize each of the built-in development devices for downsizing the copying machine, and also it is necessary to downsize each of the developer carriers provided in each of the development devices for downsizing each of the development devices. In this case, if the developer carrier is downsized, the following problems occur.
1) A high magnetic force (generally, 100 mT or more on the developer carrier) is required for the development main magnetic pole and the adjacent magnetic poles of the magnetic roller, in order to prevent the adhesion of the developer onto the photoreceptor drum, but the volume of the magnetic roller decreases in the downsized developer carrier. Therefore, it is difficult to obtain a high magnetic force.
2) In the case of a developer carrier having a reduced diameter, if the sandblast process conventionally used as the surface treatment method of the development sleeve is conducted, the development sleeve often deforms because the rigidity of the development sleeve is low. Therefore, it is difficult to obtain a shape of the developer carrier with high accuracy.
3) In the case of a developer carrier having a reduced diameter, the magnetic force change by the distance from the surface of the developer carrier increases. Therefore, it is difficult to stably attach the developer onto the developer carrier.
With respect to the above problems, a method of artificially conducting multi-pole orientation so as to enable magnetic pole formation of a multi-pole arrangement with an integral structure is proposed as described in JP H05-033802B, for example. However, with this method, there is a problem in that only about 90 mT of the magnetic force of the main magnetic pole is obtained on the developer carrier. There is also a problem in that the die structure becomes complex because the artificial multi-pole structure is adopted.
Moreover, a structure in which a magnetic block is attached to a part of a magnetic roller including an isotopic ferrite plastic magnet is proposed as described in JP2000-068120A. However, with this structure, it is difficult to achieve the magnetic flux density required for a magnetic pole except for the development main magnetic pole. For this reason, there is a problem in that this structure is not suitable for a two-component development device, and it is difficult for the above-described structure to be used for a color electrophotographic apparatus.
Furthermore, according to the invention described in JP3989180B, the present inventors propose a method of molding a plastic magnet into a pipe shape by means of extrusion molding, inserting a cored bar into a hollow part, and burying a rare-earth magnet in the circumferential face. In this case, if the outer diameter of the magnetic roller is reduced for downsizing, a sufficient volume of the magnet can not be obtained. Therefore, there is a problem in that it is difficult to obtain a high magnetic force.